The use of aqueous-based inks for ink-jet printers is well-known. Such compositions are relatively inexpensive and easy to prepare; typically, the ink comprises water and a non-aqueous component such as a glycol ether, usually diethylene glycol, and a dye. Commonly, the water and glycol ether are present in generally the same proportion and the dye is desirably present up to about 6% (w/v) of the total composition, depending on the desired optical density of the print. The dye is usually an anionic dye commonly known as an acid or direct dye. Such anionic dyes typically include sulfonate (SO.sub.3.sup.-) or carboxylate (CO.sub.2.sup.-) groups, with which are associated alkali metal cations, such as sodium (Na.sup.+).
The ink-jet printers which employ the foregoing ink compositions include an orifice, through which the ink is "jetted" or propelled onto a substrate, such as paper, mylar, and the like, and a reservoir of ink, from which ink is fed to the orifice.
The ink may be propelled by piezoelectric means or by thermal means. In the latter case, a hot resistor is used to rapidly vaporize a solvent, thereby causing the ejection of ink droplets toward the substrate.
Problems that occur with inks used in ink-jet printing include crusting, adverse reaction with nickel (used in the orifice), and kogation (a coined term unique to thermal ink-jet printing).
One mechanism by which crusting occurs is via a decrease in the water concentration of the ink in the vicinity of the orifice of the ink-jet printer due to evaporation, which results in the "salting out" of the dye, commonly known as "crusting", and causes blocking of the orifice. As a consequence, reduced print quality reliability is obtained.
Exposure of some inks to the nickel orifice results in fading of the first few printed characters upon exposure to light. Although the problem disappears after the printing of a few characters, it will reoccur with subsequent printing sessions spaced sufficiently apart in time.
Kogation occurs with thermal ink-jet printers and involves a thermally-induced decomposition product build-up on the hot resistor used to eject the ink droplets toward the substrate.
Attempts have been made to reduce or eliminate one or more of these problems. While many of these attempts have achieved a degree of success, a need remains to provide an improved ink for ink-jet printers.